Raid 10 Reads Optimized for Solid State Drives

ABSTRACT

A mechanism is provided in a data processing system. The mechanism determines a maximum queue depth of a queue for each solid state drive in a plurality of solid state drives. A given data block is mirrored between a group of solid state drives within the plurality of solid state drives. The mechanism tracks outstanding input/output operations in a queue for each of the plurality of solid state drives. For a given read operation to read the given data block, the mechanism identifies a solid state drive within the group of solid state drives based on a number of empty slots in the queue of each solid state drive within the group of solid state drives.

BACKGROUND

The present application relates generally to an improved data processingapparatus and method and more specifically to mechanisms for optimizingredundant array of independent disks reads for solid state drives.

RAID (redundant array of independent disks) is a storage technology thatcombines multiple disk drive components into a logical unit. A RAIDcontroller distributes data across the drives in one of several wayscalled “RAID levels,” depending on what level of redundancy andperformance is desired. RAID is now used as an umbrella term forcomputer data storage schemes that can divide and replicate data amongmultiple physical drives: RAID is an example of storage virtualizationand the array can be accessed by the operating system as one singledrive. The different schemes or architectures are named by the word RAIDfollowed by a number (e.g., RAID 0, RAID 1). Each scheme provides adifferent balance between the key goals: reliability, availability,performance, and capacity.

RAID 0 (block-level striping without parity or mirroring) has noredundancy. It provides improved performance and additional storage butno fault tolerance. Any drive failure destroys the array, and thelikelihood of failure increases with more drives in the array. A singledrive failure destroys the entire array because when data is written toa RAID 0 volume, the data is broken into fragments called blocks. Thenumber of blocks is dictated by the stripe size, which is aconfiguration parameter of the array. The blocks are written to theirrespective drives simultaneously. This allows smaller sections of theentire chunk of data to be read off each drive in parallel, increasingbandwidthMore drives in the array means higher bandwidth, but greaterrisk of data loss.

In RAID 1 (mirroring without parity or striping), data is writtenidentically to two drives, thereby producing a “mirrored set”; the readrequest is serviced by either of the two drives containing the requesteddata. One implementation for a system of hard disk drives is to selectthe copy that involves least seek time plus rotational latency.Similarly, a write request updates both drives. The write performancedepends on the slower of the two writes; at least two drives arerequired to constitute such an array. While more constituent drives maybe employed, many implementations deal with a maximum of only two. Thearray continues to operate as long as at least one drive is functioning.With appropriate operating system support, there can be increased readperformance, and only a minimal write performance reduction;implementing RAID 1 with a separate controller for each drive in orderto perform simultaneous reads (and writes) is sometimes called“multiplexing” (or “duplexing” when there are only two drives).

in RAID 10 (mirroring and striping), data is written in stripes acrossprimary disks that have been mirrored to secondary disks. A typical RAID10 configuration consists of four drives, two for striping and two formirroring. A RAID 10 configuration takes the best concepts of RAID 0 andRAID 1, and combines them to provide better performance and reliability.RAID 10 is often referred to as RAID 1+0 (mirrored+striped).

SUMMARY

In one illustrative embodiment, a method, in a data processing system.The method comprises determining a maximum queue depth of a queue foreach solid state drive in a plurality of solid state drives. A givendata block is mirrored between a group of solid state drives within theplurality of solid state drives. The method further comprises trackingoutstanding input/output operations in a queue for each of the pluralityof solid state drives. The method further comprises for a given readoperation to read the given data block, identifying a solid state drivewithin the group of solid state drives based on a number of empty slotsin the queue of each solid state drive within the group of solid statedrives.

In other illustrative embodiments, a computer program product comprisinga computer useable or readable medium having a computer readable programis provided. The computer readable program, when executed on a computingdevice, causes the computing device to perform various ones of, andcombinations of, the operations outlined above with regard to the methodillustrative embodiment.

In yet another illustrative embodiment, a system/apparatus is provided.The system/apparatus may comprise one or more processors and a memorycoupled to the one or more processors. The memory may compriseinstructions which, when executed by the one or more processors, causethe one or more processors to perform various ones of, and combinationsof, the operations outlined above with regard to the method illustrativeembodiment.

These and other features and advantages of the present invention will bedescribed in, or will become apparent to those of ordinary skill in theart in view of, the following detailed description of the exampleembodiments of the present invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention, as well as a preferred mode of use and further objectivesand advantages thereof, will best be understood by reference to thefollowing detailed description of illustrative embodiments when read inconjunction with the accompanying drawings, wherein:

FIG. 1 depicts a pictorial representation of an example distributed dataprocessing system in which aspects of the illustrative embodiments maybe implemented;

FIG. 2 is a block diagram of an example data processing system in whichaspects of the illustrative embodiments may be implemented;

FIG. 3A is a block diagram of a redundant array of independent disksconfiguration with optimized reads in accordance with an illustrativeembodiment;

FIG. 3B illustrates queue management for redundant array of independentdisks read and write operations in accordance with an illustrativeembodiment;

FIG. 4 is a flowchart illustrating operation of a redundant array ofindependent disks controller in accordance with an illustrativeembodiment; and

FIG. 5 is a flowchart illustrating operation of a solid state drive inaccordance with an illustrative embodiment.

DETAILED DESCRIPTION

The illustrative embodiments provide a mechanism for optimizingredundant array of independent disks reads for solid state drives. Themechanism issues reads based on which solid state drive has more room inits queue. The mechanism issues reads and writes and keeps track of howmany operations are outstanding. The mechanism determines a queue depthfor each drive. When a read is to be issued, the mechanism selects asolid state drive that has the most outstanding slots in its queue.

The illustrative embodiments may be utilized in many different types ofdata processing environments. In order to provide a context for thedescription of the specific elements and functionality of theillustrative embodiments, FIGS. 1 and 2 are provided hereafter asexample environments in which aspects of the illustrative embodimentsmay be implemented. It should be appreciated that FIGS. 1 and 2 are onlyexamples and are not intended to assert or imply any limitation withregard to the environments in which aspects or embodiments of thepresent invention may be implemented. Many modifications to the depictedenvironments may be made without departing from the spirit and scope ofthe present invention.

FIG. 1 depicts a pictorial representation of an example distributed dataprocessing system in which aspects of the illustrative embodiments maybe implemented. Distributed data processing system 100 may include anetwork of computers in which aspects of the illustrative embodimentsmay be implemented. The distributed data processing system 100 containsat least one network 102, which is the medium used to providecommunication Links between various devices and computers connectedtogether within distributed data processing system 100. The network 102may include connections, such as wire, wireless communication links, orfiber optic cables.

In the depicted example, server 104 and server 106 are connected tonetwork 102 along with storage unit 108. In addition, clients 110, 112,and 114 are also connected to network 102. These clients 110, 112, and114 may be, for example, personal computers, network computers, or thelike. In the depicted example, server 104 provides data, such as bootfiles, operating system images, and applications to the clients 110,112, and 114. Clients 110, 112, and 114 are clients to server 104 in thedepicted example. Distributed data processing system 100 may includeadditional servers, clients, and other devices not shown.

In the depicted example, distributed data processing system 100 is theInternet with network 102 representing a worldwide collection ofnetworks and gateways that use the Transmission ControlProtocol/Internet Protocol (TCP/IP) suite of protocols to communicatewith one another. At the heart of the Internet is a backbone ofhigh-speed data communication lines between major nodes or hostcomputers, consisting of thousands of commercial, governmental,educational and other computer systems that route data and messages. Ofcourse, the distributed data processing system 100 may also beimplemented to include a number of different types of networks, such asfor example, intranet, a local area network (LAN), a wide area network(WAN), or the like. As stated above, FIG. 1 is intended as an example,not as an architectural limitation for different embodiments of thepresent invention, and therefore, the particular elements shown in FIG.1 should not be considered limiting with regard to the environments inwhich the illustrative embodiments of the present invention may beimplemented.

FIG. 2 is a block diagram of an example data processing system in whichaspects of the illustrative embodiments may be implemented. Dataprocessing system 200 is an example of a computer, such as client 110 inFIG. 1, in which computer usable code or instructions implementing theprocesses for illustrative embodiments of the present invention may belocated.

In the depicted example, data processing system 200 employs a hubarchitecture including north bridge and memory controller hub (NB/MCH)202 and south bridge and input/output (I/O) controller hub (SB/ICH) 204.Processing unit 206, main memory 208, and graphics processor 210 areconnected to NB/MCH 202. Graphics processor 210 may be connected toNB/MCH 202 through an accelerated graphics port (AGP).

In the depicted example, local area network (LAN) adapter 212 connectsto SB/ICH 204. Audio adapter 216, keyboard and mouse adapter 220, modem222, read only memory (ROM) 224, hard disk drive (HDD) 226, CD-ROM drive230, universal serial bus (USB) ports and other communication ports 232,and PCI/PCIe devices 234 connect to SB/ICH 204 through bus 238 and bus240. PCI/PCIe devices may include, for example, Ethernet adapters,add-in cards, and PC cards for notebook computers. PCI uses a card buscontroller, while PCIe does not, ROM 224 may be, for example, a flashbasic input/output system (BIOS).

HDD 226 and CD-ROM drive 230 connect to SB/ICH 204 through bus 240. HDD226 and CD-ROM drive 230 may use, for example, an integrated driveelectronics (IDE) or serial advanced technology attachment (SATA)interface. Super I/O (SIO) device 236 may be connected to SB/ICH 204.

An operating system runs on processing unit 206. The operating systemcoordinates and provides control of various components within the dataprocessing system 200 in FIG. 2. As a client, the operating system maybe a commercially available operating system such as Microsoft Windows 7(Microsoft and Windows are trademarks of Microsoft Corporation in theUnited States, other countries, or both). An object-oriented programmingsystem, such as the Java programming system, may run in conjunction withthe operating system and provides calls to the operating system fromJava programs or applications executing on data processing system 200(Java is a trademark of Oracle and/or its affiliates.).

As a server, data processing system 200 may be, for example, an IBM®eServer™ System p® computer system, running the Advanced InteractiveExecutive (AIX®) operating system or the LINUX operating system (IBM,eServer, System p, and AIX are trademarks of International BusinessMachines Corporation in the United States, other countries, or both, andLIMA is a registered trademark of Linus Torvalds in the United States,other countries, or both). Data processing system 200 may be a symmetricmultiprocessor (SMP) system including a plurality of processors inprocessing unit 206. Alternatively, a single processor system may beemployed.

Instructions for the operating system, the object-oriented programmingsystem, and applications or programs are located on storage devices,such as HDD 226, and may be loaded into main memory 208 for execution byprocessing unit 206. The processes for illustrative embodiments of thepresent invention may be performed by processing unit 206 using computerusable program code, which may be located in a memory such as, forexample, main memory 208, ROM 224, or in one or more peripheral devices226 and 230, for example.

A bus system, such as bus 238 or bus 240 as shown in FIG. 2, may becomprised of one or more buses. Of course, the bus system may beimplemented using any type of communication fabric or architecture thatprovides for a transfer of data between different components or devicesattached to the fabric or architecture. A communication unit, such asmodem 222 or network adapter 212 of FIG. 2, may include one or moredevices used to transmit and receive data A memory may be, for example,main memory 208, ROM 224, or a cache such as found in NB/MCH 202 in FIG.2.

Those of ordinary skill in the art will appreciate that the hardware inFIGS. 1 and 2 may vary depending on the implementation. Other internalhardware or peripheral devices, such as flash memory, equivalentnon-volatile memory, or optical disk drives and the like, may be used inaddition to or in place of the hardware depicted in FIGS. 1 and 2. Also,the processes of the illustrative embodiments may be applied to amultiprocessor data processing system, other than the SMP systemmentioned previously, without departing from the spirit and scope of thepresent invention.

Moreover, the data processing system 200 may take the form of any of anumber of different data processing systems including client computingdevices, server computing devices, a tablet computer, laptop computer,telephone or other communication device, a personal digital assistant(PDA), or the like. In some illustrative examples, data processingsystem 200 may be a portable computing device that is configured withflash memory to provide non-volatile memory for storing operating systemfiles and/or user-generated data, for example. Essentially, dataprocessing system 200 may be any known or later developed dataprocessing system without architectural limitation.

FIG. 3A is a block diagram of a redundant array of independent disksconfiguration with optimized reads in accordance with an illustrativeembodiment. The redundant array of independent disks (RAID)configuration is a RAID 10 configuration with mirroring and striping.RAID controller 310 stripes data blocks across drive groups and mirrorsdata blocks within a drive group. For example, RAID controller 310 maydivide data to be written into data blocks A, B, and C. RAID controller310 then writes data block A to group 1, writes data block B to group 2,and writes data block C to group 3. Group 1 includes drive 1 311 anddrive 2 312; group 2 includes drive 3 313 and drive 4 314; group 3includes drive 5 315 and drive 6 316.

RAID 10 requires at least four drives: two drives for striping and twodrives for mirroring. RAID 10 may include more than four drives—usuallyan even number—depending on how many blocks, or stripes, are to bewritten for each unit of data. In the depicted example, each unit ofdata is divided into three stripes. Conceivably, a RAID 10 configurationmay also include more than one mirror, resulting in more than two drivesin a group; however, a typical RAID 10 configuration will include twodrives per group.

RAID controller 310 may be a hardware controller, such as a storagecontroller in a standalone computer system, a server, or a storageenclosure. Alternatively, RAID controller 310 may be a storagecontroller card or blade in a server chassis. In another alternativeembodiment, RAID controller 310 may be RAID software or firmwareexecuting within a desktop or server computer environment or within avirtual machine manager.

RAID 10 is often used for critical performance applications. In fact,RAID 10 is often the RAID level to use with guaranteed response timeofferings. RAID 10 accelerates read operations, because the RAIDcontroller may distribute read operations between multiple drives. RAID10 algorithms have been developed over the years for hard disk drives(HDDs). However, with HDDs, RAID controller 310 would typicallyalternate read operations between the pair of drives (e.g., drive 313and drive 314), or selects a HDD to which to issue a read operationbased on how far the HDD has to seek.

A solid-state drive (SSD) is a data storage device that uses integratedcircuit assemblies as memory to store data persistently. SSD technologyuses electronic interfaces compatible with traditional blockinput/output (I/O) hard disk drives. SSDs do not employ any movingmechanical components, which distinguishes them from traditionalmagnetic disks such as hard disk drives (HDDs) or floppy disks, whichare electromechanical devices containing spinning disks and movableread/write heads. Compared with electromechanical disks, SSDs aretypically less susceptible to physical shock and are usually silent.More importantly, SSDs have lower access time and latency because theyhave no seek time. For this reason, existing RAID 10 algorithms may notbe optimal for configurations with SSDs.

In accordance with the illustrative embodiment, drives 311-316 are solidstate drives. RAID controller 310 issues read operations based on whichsolid state drive has more room in its queue. Since SSDs must performgarbage collection to reclaim space, various amounts of backgroundactivity may exist in the mirrored pairs. By examining queue depth, itis possible to determine which drive is the least busy at the currenttime. RAID controller 310 issues reads and writes and keeps track of howmany operations are outstanding. RAID controller 310 determines a queuedepth for each drive. When a read operation is to be issued, RAIDcontroller 310 selects a solid state drive that has the most outstandingslots in its queue.

Because reads and writes are issued to SSDs at the same time and theyhave the potential to do so many more operations, the response time canbe skewed if one device in a RAID 10 pair is having to do more readoperations or is performing more background operations than the otherdevice in the pair. Skew is not only caused by having more readoperations issued, but may be caused by reaching the maximum queue depthof a particular drive and therefore either queuing read operationswithin the controller 310 or, worse, not being able to accept any morereads from the requesting host.

FIG. 3B illustrates queue management for redundant array of independentdisks read and write operations in accordance with an illustrativeembodiment. RAID controller 310 maintains a queue 321-326 for eachdrive. RAID controller 310 selects a maximum queue depth for each drivedepending on a number of factors, many of which are outside the scope ofthis description. RAID controller 310 issues reads based on which drivehas more room in its queue. For example, when reading from group 1, RAIDcontroller 310 issues the read operation to a drive selected betweendrive 1 311 and drive 2 312 based on which drive has the mostoutstanding slots in its queue. In the depicted example, queue 322 hasmore outstanding slots than queue 321; therefore, RAID controller 310issues the read operation to drive 2 312.

Thus, RAID controller 310 determines the drive must be capable ofhandling the read operations. This results in keeping overall responsetimes optimally low because skew is kept to a minimum. If one drive isskewed, response time for its reads will be determined by the elapsedtime involved in cycling through items on the queue. Therefore, longresponse times can start to develop even if some devices are relativelyidle. Even though the illustrative embodiment concerns read operations,RAID controller 310 takes both read operations and write operations intoaccount when issuing reads. RAID controller 310 calculates empty slotsby taken maximum operations as follows:

empty_slots=(reads+writes)−(reads_issued+writes_issued)

wherein (reads+writes) is the maximum number of operations in themaximum queue depth and (reads_issued+writes_issued) is the total numberof operations issued to the SSD.

In accordance with one aspect of the illustrative embodiment, certainprocesses in an SSD may cause the SSD to return a max queue depthadjustment number in a status message. A max queue depth adjustment of 0(zero) indicates no adjustment to the queue at all. A max queue depthadjustment of −1 indicates to reduce by one the max queue depth. Forinstance, if the SSD detects a background process, such as read disturbmitigation, then the SSD returns a max queue depth adjustment of −1 forthat drive. The RAID controller 310 examines the difference between SSDsin a pair to adjust what the maximum operations for a drive should beand which SSD should be issued a read operation. When a backgroundprocess completes, the SSD may return a max depth adjustment of 0 toindicate that reducing queue depth is not necessary.

Conditions that can result in a maximum queue depth reduction are asfollows:

1. Internal RAID rebuild inside the drive.

2. Switching from mostly sequential to mostly random operations, meaningthat due to preconditioning, the drive may less than optimal for a

3. A high number of relocates due to read disturb storms.

4. Block retirement reaching a certain level.

5. Free blocks for garbage collection reaching a critical point.

As wilt be appreciated by one skilled in the art, the present inventionmay be embodied as a system, method, or computer program product.Accordingly, aspects of the present invention may take the form of anentirety hardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module,” or “system.” Furthermore,aspects of the present invention may take the form of a computer programproduct embodied in any one or more computer readable medium(s) havingcomputer usable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CDROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, in abaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Computer code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, radio frequency (RF), etc., or anysuitable combination thereof.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java™, Smalltalk™, C++, or the like, and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer, or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to the illustrativeembodiments of the invention. It will be understood that each block ofthe flowchart illustrations and/or block diagrams, and combinations ofblocks in the flowchart illustrations and/or block diagrams, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer or other programmabledata processing apparatus, create means for implementing thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions thatimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus, or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

FIG. 4 is a flowchart illustrating operation of a redundant array ofindependent disks controller in accordance with an illustrativeembodiment. Operation begins (block 400) and the redundant array ofindependent disk (RAID) controller determines whether a status messageis received from a solid state drive (SSD) (block 401). If a statusmessage is received, the RAID controller determines a queue depth forthe SSD (block 402). The RAID controller may determine a maximum queuedepth based on a number of factors. More particularly, the RAIDcontroller may determine the maximum queue depth for the SSD based on amaximum queue depth adjustment number received in the status message.

Thereafter, or if a status message is not received in block 401, theRAID controller determines whether a write operation is to be performed(block 403). If a write operation is to be performed to write a datablock for a stripe, the RAID controller issues the write operation toall SSDs in a selected group (block 404).

Thereafter, or if a write operation is not to be performed in block 403,the RAID controller determines whether a read operation is to beperformed (block 405). If a read operation is to be performed to read adata block, the RAID controller identifies a SSD with more empty slotsin its queue (block 406) and issues the read to the identified SSD(block 407). Thereafter, or if a read operation is not to be issued inblock 405, operation returns to block 401 to determine whether a statusmessage is received from an SSD.

FIG. 5 is a flowchart illustrating operation of a solid state drive inaccordance with an illustrative embodiment. Operation begins (block500), and the solid state drive (SSD) determines whether a statusmessage is to be sent to the RAID controller (block 501). If a statusmessage is not to be sent, operation returns to block 501 until a statusmessage is to be sent to the RAID controller.

If a status message is to be sent to the RAID controller in block 501,the SSD determines a maximum queue depth adjustment number (block 502)and returns the status message with the maximum queue depth adjustmentnumber (block 503). Thereafter, operation returns to block 501 todetermine whether a status message is to be sent to the RAID controller.

In accordance with one aspect of the illustrative embodiment, certainprocesses in an SSD may cause the SSD to return a max queue depthadjustment number in a status message. A max queue depth adjustment of 0(zero) indicates no adjustment to the queue at all. A max queue depthadjustment of −1 indicates to reduce by one the max queue depth. Forinstance, if the SSD detects a background process, such as read disturbmitigation, then the SSD returns a max queue depth adjustment of −1 forthat drive. When a background process completes, the SSD may return amax depth adjustment of 0 to indicate that reducing queue depth is notnecessary.

Conditions that can result in a maximum queue depth reduction are asfollows:

1. Internal RAID rebuild inside the drive.

2. Switching from mostly sequential to mostly random operations, meaningthat due to preconditioning, the drive may less than optimal for awhile.

3. A high number of relocates due to read disturb storms.

4. Block retirement reaching a certain level.

5. Free blocks for garbage collection reaching a critical point.

The flowchart and block diagrams in the figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

Thus, the illustrative embodiments provide a mechanism for optimizingredundant array of independent disks reads for solid state drives. Themechanism issues reads based on which solid state drive has more room inits queue. The mechanism issues reads and writes and keeps track of howmany operations are outstanding. The mechanism determines a queue depthfor each drive. When a read is to be issued, the mechanism selects asolid state drive that has outstanding slots in its queue.

As noted above, it should be appreciated that the illustrativeembodiments may take the form of an entirety hardware embodiment, anentirely software embodiment or an embodiment containing both hardwareand software elements. In one example embodiment, the mechanisms of theillustrative embodiments are implemented in software or program code,which includes but is not limited to firmware, resident software,microcode, etc.

A data processing system suitable for storing and/or executing programcode will include at least one processor coupled directly or indirectlyto memory elements through a system bus. The memory elements can includelocal memory employed during actual execution of the program code, hulkstorage, and cache memories which provide temporary storage of at leastsome program code in order to reduce the number of times code must beretrieved from bulk storage during execution.

Input/output or I/O devices (including but not limited to keyboards,displays, pointing devices, etc.) can be coupled to the system eitherdirectly or through intervening I/O controllers, Network adapters mayalso be coupled to the system to enable the data processing system tobecome coupled to other data processing systems or remote printers orstorage devices through intervening private or public networks. Modems,cable modems and Ethernet cards are just a few of the currentlyavailable types of network adapters.

The description of the present invention has been presented for purposesof illustration and description, and is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain theprinciples of the invention, the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method, in a data processing system,comprising: determining a maximum queue depth of a queue for each solidstate drive in a plurality of solid state drives, wherein a given datablock is mirrored between a group of solid state drives within theplurality of solid state drives; tracking outstanding input/outputoperations in a queue for each of the plurality of solid state drives;and for a given read operation to read the given data block, identifyinga solid state drive within the group of solid state drives based on anumber of empty slots in the queue of each solid state drive within thegroup of solid state drives.
 2. The method of claim 1, wherein the datablock is a stripe of data being written to the plurality of solid statedrives, wherein the plurality of solid state drives are configured as aredundant array of independent disks with mirroring and striping.
 3. Themethod of claim 1, further comprising: receiving a maximum queue depthadjustment value from a given solid state drive; and determining themaximum queue depth for the given solid state drive based on the maximumqueue depth adjustment value.
 4. The method of claim 3, wherein themaximum queue depth adjustment value is received in a status messagefrom the given solid state drive.
 5. The method of claim 3, wherein thegiven solid state drive sets the maximum queue depth adjustment valueresponsive to at least one of an internal redundant array of independentdisks rebuild inside the given solid state drive, switching from mostlysequential to mostly random operations, a predetermined number ofrelocates due to read disturb storms, block retirement reaching apredetermined level, or free blocks for garbage collection reaching acritical point.
 6. The method of claim 1, further comprising:determining the number of empty slots in the queue of each solid statedrive, comprising subtracting the total number of read and writeoperations issued to each given solid state drive from the maximum queuedepth of the given solid state drive.
 7. A computer program productcomprising a computer readable storage medium having a computer readableprogram stored therein, wherein the computer readable program, whenexecuted on a computing device, causes the computing device to:determine a maximum queue depth of a queue for each solid state drive ina plurality of solid state drives, wherein a given data block ismirrored between a group of solid state drives within the plurality ofsolid state drives; track outstanding input/output operations in a queuefor each of the plurality of solid state drives; and for a given readoperation to read the given data block, identify a solid state drivewithin the group of solid state drives based on a number of empty slotsin the queue of each solid state drive within the group of solid statedrives.
 8. The computer program product of claim 7, wherein the datablock is a stripe of data being written to the plurality of solid statedrives, wherein the plurality of solid state drives are configured as aredundant array of independent disks with minoring and striping.
 9. Thecomputer program product of claim 7, wherein the computer readableprogram further causes the computing device to: receive a maximum queuedepth adjustment value from a given solid state drive; and determine themaximum queue depth for the given solid state rive based on the maximumqueue depth adjustment value.
 10. The computer program product of claim9, wherein the maximum queue depth adjustment value is received in astatus message from the given solid state drive.
 11. The computerprogram product of claim 9, wherein the given solid state drive sets themaximum queue depth adjustment value responsive to at least one of aninternal redundant array of independent disks rebuild inside the givensolid state drive, switching from mostly sequential to mostly randomoperations, a predetermined number of relocates due to read disturbstorms, block retirement reaching a predetermined level, or free blocksfor garbage collection reaching a critical point.
 12. The computerprogram product of claim 7, wherein the computer readable programfurther causes the computing device to: determine the number of emptyslots in the queue of each solid state drive, comprising subtracting thetotal number of read and write operations issued to each given solidstate drive from the maximum queue depth of the given solid state drive.13. The computer program product of claim 7, wherein the computerreadable program is stored in a computer readable storage medium in adata processing system and wherein the computer readable program wasdownloaded over a network from a remote data processing system.
 14. Thecomputer program product of claim 7, wherein the computer readableprogram is stored in a computer readable storage medium in a server dataprocessing system and wherein the computer readable program isdownloaded over a network to a remote data processing system for use ina computer readable storage medium with the remote system.
 15. Anapparatus, comprising: a processor; and a memory coupled to theprocessor, wherein the memory comprises instructions which, whenexecuted by the processor, cause the processor to: determine a maximumqueue depth of a queue for each solid state drive in a plurality ofsolid state drives, wherein a given data block is mirrored between agroup of solid state drives within the plurality of solid state drives;track outstanding input/output operations in a queue for each of theplurality of solid state drives; and for a given read operation to readthe given data block, identify a solid state drive within the group ofsolid state drives based on a number of empty slots in the queue of eachsolid state drive within the group of solid state drives.
 16. Theapparatus of claim 15, wherein the data block is a stripe of data beingwritten to the plurality of solid state drives, wherein the plurality ofsolid state drives are configured as a redundant array of independentdisks with minoring and striping.
 17. The apparatus of claim 15, whereinthe instructions further cause the processor to: receive a maximum queuedepth adjustment value from a given solid state drive; and determine themaximum queue depth for the given solid state drive based on the maximumqueue depth adjustment value.
 18. The apparatus of claim 17, wherein themaximum queue depth adjustment value is received in a status messagefrom the given solid state drive.
 19. The apparatus of claim 17, whereinthe given solid state drive sets the maximum queue depth adjustmentvalue responsive to at least one of an internal redundant array ofindependent disks rebuild inside the given solid state drive, switchingfrom mostly sequential to mostly random operations, a predeterminednumber of relocates due to read disturb storms, block retirementreaching a predetermined level, or free blocks for garbage collectionreaching a critical point.
 20. The apparatus of claim 15, wherein theinstructions further cause the processor to: determine the number ofempty slots in the queue of each solid state drive, comprisingsubtracting the total number of read and write operations issued to eachgiven solid state drive from the maximum queue depth of the given solidstate drive.